Optical scanning device

ABSTRACT

An optical scanning device incorporating a compound objective lens  10  and a radiation detector  31  is described for reading an optical record carrier  1  in which the optical record carrier  1  is provided with a transparent layer  2 . The thickness of the transparent layer  2  and the effective radius of the detector  31  are adapted such that the signal-to-noise ratio of the device is significantly improved and/or local heating of an air gap below an air slider is reduced. The optical record carrier is provided with a lubricant to facilitate the use of a slider optical head  40.

This invention relates to an optical scanning device for scanning anoptical record carrier, such as an optical disk, including at least oneinformation layer. The device includes a radiation source for generatinga radiation beam, a compound objective lens system located in an opticalpath between the radiation source and the record carrier for convergingthe radiation beam to a spot on an information layer and a slider havingan optically transparent part for transmitting radiation from theradiation source. The invention also relates to an optical recordcarrier for use in such an optical scanning device.

There is a need for the production of optical scanning devices capableof reading high capacity record carriers. For example, high capacityoptical disks have been devised that have multiple information layers ina single disk. Furthermore, it is also possible to increase the amountof data stored on such a disk by decreasing the size of the datamarkings on the disk. In order to read such data accurately, opticalscanning devices using a relatively short wavelength and a highnumerical aperture (NA) objective lens system with at least say NA>1 aredesirable.

In one solution to achieve NA>1, the compound lens system includes amulti-lens objective, where one of the lens elements is a second,substantially plano-convex, lens placed on a slider which allows thelens to fly at a height within a wavelength λ of the surface of thedisk, to provide an evanescent coupling between the second lens and thedisk for the radiation, a so-called near-field arrangement. The lenstypically flies at a height of ⅕λ (generally less than 0.1 μm). Such alens is referred to herein as a solid immersion lens (SIL). Themulti-lens objective will include a further lens element, such as asingle lens well corrected for aberrations. See, for exampleEP-A-0,867,873 and JP-A-09161311.

In the case of EP-A-0,867,873 a record carrier uses a transparent layerhaving a thickness between 3 and 177 μm to cover the information layer,where the thickness is selected to aid the increase in storage capacityof the optical disk. No evanescent coupling is used to couple radiationinto the record carrier; i.e. the record carrier is used in a far-fieldarrangement.

In the above disclosed systems, the radiation is focussed on theinformation layer of the optical disk and the reflected radiation istransmitted back through the compound objective lens system towards asuitable detection system where the optical signal is converted intoelectrical signals representative of the data stored in the informationlayer on the optical disk. A problem with this system is that straylight at the detector reduces the signal to noise ratio.

A further problem in a magneto-optic system relates to the positioningof the magnetic coil in relation to the information layer of the disk.Namely, the coil must be placed sufficiently close to the informationlayer to allow sufficient magnetic modulation at the information layerduring recording and to allow a desired data scanning rate. This problemoccurs in a so-called far-field arrangement, where there is noevanescent coupling between the lower surface of the objective lenssystem and the top surface of the disk.

In EP-A-0,878,793 a magneto-optic system is described in which amagnetic coil is placed on the same side of the disk as the objectivelens system, and the information layer on the disk is placed behind aprotective transparent layer of between 0 and 100 μm thickness. Thedistance between the magnetic coil and the optical disk is preferablyset at least 20 μm.

According to an aspect of the invention there is provided An opticalscanning device adapted to be used for scanning an optical recordcarrier when located in a scanning position in the device, the opticalrecord carrier including an information layer for storing data and atransparent layer through which radiation passes to reach theinformation layer, the device including a radiation source forgenerating a radiation beam, an objective lens system located in thepath of the radiation beam from the radiation source to the scanningposition of the optical record carrier to provide for an evanescentcoupling of the radiation beam with the optical record carrier, and aradiation detector for detecting radiation reflected by the opticalrecord carrier, wherein the detector is configured, in combination witha thickness of the transparent layer, such that a significant proportionof stray reflected light, during scanning, falls outside an effectivepart of the detector, whilst substantially all radiation reflected bythe information layer falls inside the effective part of said detector.

In prior art systems a portion of the radiation transmitted from theradiation source towards the optical disk is reflected from the air gapback into the optical path without having reached the information layerof the optical disk. As the air gap is small, the radiation reflectedfrom the air gap will follow substantially the same path as theradiation reflected from the information layer of the optical disk andas such, will be transmitted back through the compound objective lenssystem and fall onto the detection system. This will adversely affectthe signal-to-noise ratio (SNR) of the optical scanning system, and willlead to poor reproduction of the information stored on the disk.

The present invention is capable of providing a significant improvementin the SNR during the scanning of an optical record carrier using thedevice, by preventing stray radiation reflected from the gap impingingon the effective part of the detector. Whilst the term “significantproportion” is intended to include arrangements providing a relativelysmall amount of reduction in the stray radiation (say 10% or more),larger reductions, above 50% are preferred. Indeed, it is possible toreduce the amount of stray radiation by over 90%, as will be describedin further detail below. At the same time, it is possible to ensure that“substantially all” the wanted radiation does follow a path such that itimpinges on the effective part of the detector. Whilst some loss of thewanted radiation is unavoidable in the optical system due to e.g.absorption, reflection and limitation in detector efficiency, this lossis mostly not due to the locational relationship of the path of thewanted radiation with the detector, which may remain optimized inembodiments of the invention.

This invention is applicable in the case of both near-field andfar-field arrangements. However, the invention is particularlyapplicable in the case of near-field arrangements. In this case anyreduction in the efficiency of the near-field coupling, due for exampleto flying height variations, can cause total internal reflection at theinterface, whereby significant amounts of stray light can be generated.

In prior art systems coherent cross-talk between the signal layer andthe radiation reflected from the air gap can cause significantdeterioration in the signal-to-noise ratio and servo signals. Thepresent invention is also capable of reducing such coherent cross-talk,in both the near-field and the far-field case.

To have low coherent cross-talk from the air gap it is preferable tohave the transparent layer thickness above the value:$d = \frac{20 \cdot \lambda \cdot n_{cov}}{({NA})^{2}}$where λ is the radiation wavelength, n_(cov) is the refractive index ofthe transparent layer, also called cover layer, and NA is the numericalaperture of the system at the disk.

According to a further aspect of the invention there is provided anoptical scanning device adapted to be used for scanning an opticalrecord carrier when located in a scanning position in the device, theoptical record carrier including an information layer for storing dataand a transparent layer having a thickness of between 2 and 50 μmthrough which radiation passes to reach the information layer, thedevice including a radiation source for generating a radiation beam, anobjective lens system located in the optical path between the radiationsource and the scanning position of the optical record carrier, a sliderfor flying at a height of less than 20 μm above the optical recordcarrier and having a transparent portion for transmitting said radiationbeam, and a radiation detector for detecting radiation reflected by theoptical record carrier.

An advantage of a transparent layer of between 2 and 50 μm thickness isthat it can be used to reduce the heating effect of the laser beam atthe entrance face of the record carrier, i.e. the surface through whichthe radiation enters the record carrier. The heating at the entranceface may cause unwanted flying height instabilities of the slider, andif a surface lubricant is used on the record carrier, unwanted localisedheating of the lubricant. The reduction of the heating is particularlyimportant during writing data on the record carrier, when the power inthe radiation beam is relatively high. The transparent layer places theentrance face at least one focal depth (λ/NA²) away from the focus ofthe laser beam, such that the laser beam is not so close to its smallestpossible diameter as it passes through the surface of the recordcarrier, thereby reducing the intensity of its heating effect at theentrance face.

Thus a slider, flying at a height of up to 10-20 μm above the recordcarrier surface, may also be used in far-field arrangements, inparticular in magneto-optic systems. This is also a narrow gap, allowinga magnetic coil to be held sufficiently close to the disk surface toachieve high data scanning rates, but less narrow than that required foran evanescent coupling.

According to a yet further aspect of the present invention, there isprovided an optical record carrier for use with an optical scanningdevice according to the invention, the optical record carrier beingadapted for use with radiation of a predetermined wavelength and anobjective lens system of a predetermined numerical aperture, wherein theoptical record carrier includes an information layer for storing dataand a transparent layer through which radiation passes to reach theinformation layer, the outer surface of the transparent layer having alubricant coating for improving the scanning characteristics whenscanned with a slider, in which the thickness of the transparent layeris at least ${d = \frac{5 \cdot \lambda \cdot n_{cov}}{{NA}_{NF}}},$where d is the thickness of the transparent layer, λ is the radiationwavelength, n_(cov) is the refractive index of the transparent layer,NA_(NF) is the numerical aperture of the objective lens system.

Further features and advantages of the invention will become apparentfrom the following description of preferred embodiments of theinvention, given by way of example only, with reference to theaccompanying diagrams in which:

FIG. 1 is a schematic illustration of a general layout of an opticalscanning device used in accordance with embodiments of the invention;

FIG. 2 is a cross sectional view of a compound objective lens systemincluding a detector and transparent layer used in accordance with theinvention;

FIG. 3 a shows the radiation reflected towards the detection systemgenerated by reflection of radiation from the information layer of anoptical disk using the objective lens system of FIG. 2;

FIG. 3 b is a simulation of the radiation reflected towards thedetection system generated by reflection of radiation from the air gapbetween the SIL and the optical disk using the objective lens system ofFIG. 2;

FIG. 4 shows the dependency of detector signals as a function of defocus(A₂₀) for a detector and for a number of different thickness air gaps;

FIG. 5 shows the value of cross talk for three different thickness airgaps as a function of defocus (A₂₀) of stray radiation on a relativelylarge detector; and

FIG. 6 shows a schematic side view of a compound objective lens systemmounted in an optical scanning device in accordance with an embodimentof the invention.

FIG. 1 shows a cross-section of an embodiment of an optical recordcarrier 1 in the form of a disk according to the invention, whichincludes an outer transparent layer 26 covering at least one informationlayer 3. In the case of a multilayer optical disk, two or moreinformation layers are arranged behind the transparent layer 26, atdifferent depths within the disk. The side of the information layer 3,or in the case of a multilayer disk the side of the layer furthest awayfrom the transparent layer 26, facing away from the transparent layer isprotected from environmental influences by a protection layer 4. Theside of the transparent layer facing the device is the disk entranceface 5.

Information may be stored in the information layer 3 or layers of theoptical disk 1 in the form of optically detectable marks arranged insubstantially parallel, concentric or spiral tracks, not shown inFIG. 1. The marks may be in any optically readable form, for example inthe form of pits or areas with a reflection coefficient different fromtheir surroundings, or magneto-optical form.

The optical scanning device includes a radiation source 6, for example asemiconductor laser, emitting a diverging radiation beam 7. The opticalpath includes a collimator lens 12 and a compound objective lens system10 that includes a back lens element 13 and a front lens element 14. Thefront lens element 14 is a plano-convex SL. Each of the collimator lens12 and the back lens element 13 are shown in FIG. 1 as convex lenses,however, other lens types such as plano-convex, convex-concave orconcave-convex lenses may also be used. Particular reference is made tothe arrangement shown in FIG. 2, to be described below.

The collimator lens 12 transforms the diverging radiation beam 7 to agenerally collimated beam 16. By collimated, a beam substantiallyparallel to the optical axis of the system is meant. The position of thecollimator 12 is, in this embodiment, fixed but the collimator can alsobe axially movable by means of a focus servo operation to maintainfocusing of the beam spot on the desired information layer position.

The second lens element transforms the beam 16 to a convergent beam 17between the back lens element 13 and the front lens element 14. The backlens element 13 is, in this embodiment, axially movable by means of afocus servo operation to maintain focusing of the beam spot on thedesired information layer position but can also be fixed with respect tothe front lens 14. The effect of the front lens element 14, being anSIL, is to increase the numerical aperture of the system further,without introducing spherical aberration, or reducing unwanted sphericalaberrations, as the beam enters the entrance face 5 of the disk, therebyincreasing the convergence of the beam inside the transparent layer 2compared to the effect achieved without a SIL present. The beam istherefore focussed by the compound objective lens 10 to a point on theinformation layer 3 within the optical disk 1.

The reflected beam is transformed by the compound objective lens 10 incorrespondingly opposite stages, from a greatly divergent beam 20between the front lens element 14 and the back lens element 13, to acollimated beam 21 between the back lens element 13 and the collimatorlens 12, to a convergent reflected beam 22 incident on a detector system24.

The detector system 24 captures the radiation and converts it intoelectrical signals. One of these signals is an information signal, thevalue of which represents the data read from the information layer 3 ofthe optical disk 1.

The objective lens is held close to the entrance face of the upper disk,within 1 wavelength of the radiation, generally within 1 μm by means ofan air-bearing slider (not shown) of suitable dimensions. In order toimprove the tribology of the interface, a lubricant 27 is coated on thetransparent layer 26. The entrance face of the disk is now formed by thesurface of the lubricant coating facing the front lens element 14. Thelubricant may be formed from a polyfluoropolyether, such as those knownas Fomblin™ and Zdol™, The lubricant forms a uniform layer of thicknesstypically less than 1 nm, formed by dip-coating the disk in a solutionof the lubricant. The top surface of the transparent layer may betreated with a hard coating of diamond-like carbon before theapplication of the lubricant, to improve the application of thelubricant.

FIG. 2 shows an optical scanning device including a two-lens objectivelens system 10 as described above. The collimator lens element 12 is aconvex lens; the back lens element 13 is a convex lens, and the frontlens element 14 is a spherical plano-convex SIL. The positions of theSIL 14 and the lubricant-coated transparent layer 26 together define anair gap 30.

An exemplary outer ray 8 of the radiation beam 7 in FIG. 2 enters theobjective lens system 10 and is incident on the collimator lens element12 and is refracted towards the optical axis at the point A. The ray isthen incident on the first lens element 13 and is further refractedtowards the optical axis at point B by the first lens element. The rayexits the second lens element and is incident on the entrance surface ofthe SIL 14 at point C. In this embodiment the ray is not refracted to asubstantial extent at the entrance of the SIL 14, the air gap 30 or thetransparent layer 26. It should be added that an alternativeconfiguration of SIL providing additional focusing, as is known in theart, is also applicable within the framework of the current invention.The radiation entering the objective lens system is focussed by thelenses 12, 13, 14 to a point 19 on the, or an, information layer 3 ofthe optical disk 1. The ray is then reflected by the optical disk 1.

The lenses 12, 13, 14 transform the reflected ray, in correspondinglyopposite stages towards a detector, the effective part 31 of which isshown in perspective, where the radiation is captured and converted intoelectrical signals corresponding to the data on the information layer ofthe optical disk 1.

When the radiation beam passes through the SIL 14, as describedpreviously, a portion of the incident radiation will be reflected by theair gap 30 back into the optical path and hence to the effective part 31of the detector. However, by selecting an appropriate thickness oftransparent layer 2 and a size of the detector 31, it is possible toattain de-focus of the rays reflected by the air gap 30, with respect tothe rays reflected by the information layer 3 of the optical disk 1,such that at least a significant proportion, preferably substantiallyall of the rays reflected by the air gap are not incident on thedetector 31.

The reflection of rays of the radiation beam 7 in the SIL at theinterface with the air gap 30 increases with increasing NA. Rays havingNA>1 are reflected at the interface by total internal reflection. Thepart of the radiation beam reflected by the interface, called beam part32, is shown in FIG. 2 by a hatched area. The beam part 32 is convergesto a focus point F within the SIL 14, then de-focussed by the SIL 14 andthe second lens element 13, and subsequently brought to focus by thefirst lens element 12 at a point G behind the detector 31. In the planeof the detector, beam part 32 does not impinge on the effective part 31having a radius rd of the detector (although it may impinge on part ofthe detector, such as a non-radiation-sensitive part where it does notcontribute to the electrical detector signals). In this way, thedetector 31 detects only the rays of the radiation beam reflected by theinformation layer 3 and not the rays reflected by the air gap 30. Thisimproves the signal-to-noise ratio of the detector signals.

This result may be achieved by appropriately arranging the effectivedetector radius rd and the thickness of the transparent layer 26.Namely, the thickness of the transparent layer 26 should be selected tofall within a certain range and the detector size should be limited. Thevalues of these parameters can be expressed for every optical recordingsystem in a form dependent on the exact choice of system parameters,like NA and λ.

The minimum thickness of the transparent layer 26 and the radius of thedetector 31 can be estimated as follows on the basis of ray tracecalculations.

Consider the rays in FIG. 2. Let the system have a total numericalaperture equal to NA_(NF)>1. The reflected rays are focused on adetector in a diffraction-limited image since such configuration ensuresthe best selectivity between the rays reflected from the informationcarrier and from the air gap. To avoid signal deterioration, thedetector size should be substantially larger than the spot size on thedetector 31. If the lens focusing the rays on the detector has anumerical aperture of NA_(det) the preferred detector radius rd can beestimated as: $\begin{matrix}{{\left. r_{d} \right.\sim 10}\quad\frac{\lambda}{{NA}_{\det}}} & (A)\end{matrix}$

Those rays focused on the disc whose angles of incidence θ on the airgap are within a ring defined by 1<(n sin θ)<NA_(NF) will be reflectedfrom the air gap with higher reflection coefficients and are relativelymore harmful from the point of view of signal-to-noise ratiodeterioration. ‘n’ is the refractive index of the material in which theangle θ is determined. In order that the rays of the beam part 32 justmiss the detector, these rays should converge at a point G lying adistance Δl behind the detector surface, where: $\begin{matrix}{{\Delta\quad l} = {\frac{r_{d}}{{NA}_{\det}}{NA}_{NF}}} & (B)\end{matrix}$Substituting from (A): $\begin{matrix}{{\Delta\quad l} = \frac{10 \cdot \lambda \cdot {NA}_{NF}}{\left( {NA}_{\det} \right)^{2}}} & (C)\end{matrix}$

If the transparent layer 26 is assumed to have a refractive indexn_(cov), generally not equal to the refractive index of the SIL 14,n_(SIL), a virtual source reflected from the air gap 30 will bedisplaced with respect to the spot on the information layer 3 by a valueof: ${\Delta\quad z} = {2d\quad\frac{n_{SIL}}{n_{cov}}}$where d is the thickness of the transparent layer 26.

The presence of the SIL will cause the spot to be displaced at thedetector side by:${\Delta\quad l} = {{\Delta\quad{z.n_{SIL}}\quad\frac{\left( {{NA}_{NF}/n_{SIL}} \right)^{2}}{\left( {NA}_{\det} \right)^{2}}} = {{\Delta\quad z\quad\frac{\left( {NA}_{NF} \right)^{2}}{{n_{SIL}\left( {NA}_{\det} \right)}^{2}}} = \frac{2{d\left( {NA}_{NF} \right)}^{2}}{{n_{cov}\left( {NA}_{\det} \right)}^{2}}}}$If this value of Δl is substituted into Equation (C), a value for theminimum thickness of the transparent layer 26 is obtained:$\frac{2{d\left( {NA}_{NF} \right)}^{2}}{{n_{cov}\left( {NA}_{\det} \right)}^{2}} = \frac{10 \cdot \lambda \cdot {NA}_{NF}}{\left( {NA}_{\det} \right)^{2}}$$d = \frac{5 \cdot \lambda \cdot n_{cov}}{{NA}_{NF}}$

The value of de-focus on the detector, A₂₀ (in terms of Zernikecoefficients) is given by:$A_{20} = \frac{\Delta\quad{l\left( {NA}_{\det} \right)}^{2}}{4}$so substituting for Δl given in equation (B) above: $\begin{matrix}{A_{20} = {\frac{r_{d}{NA}_{NF}}{{NA}_{\det}}\frac{\left( {NA}_{\det} \right)^{2}}{4}}} \\{A_{20} = \frac{{NA}_{\det} \cdot r_{d} \cdot {NA}_{NF}}{4}}\end{matrix}$

and substituting for rd given in equation (A) above:A₂₀˜2.5·λ·NA_(NF)For example, for NA_(NF)=1.4A₂₀˜3.52λ

The findings of this paraxial analysis are further confirmed bynumerical modelling of diffraction. FIG. 3 a shows the light intensityof a beam reflected from the information layer 3 of an optical disk 1and focused on the detector. FIG. 3 b shows the light intensitycorresponding to a ring 1<(n sin θ)<NA_(NF) with a value of 3λ de-focus(A₂₀) focused on the same detector. In this way, with a de-focus of 3λ,a detector can have a size larger than the spot shown in FIG. 3 a, butsmaller than the ring of light reflected by the air gap 30 and shown asa ring in FIG. 3 b, so as to collect only rays that have been reflectedby the information layer 3 of the optical disk 1. In this way the straybeams reflected by the air gap 30 miss the detector 31.

A more detailed analysis using an actual distribution of intensity ofthe rays reflected from a sub-wavelength air gap rather than the ringintensity distribution described above has been carried out. Using theradiation intensity distribution reflected from air gap 30 the totalintensity of radiation collected by the detector has been calculated forthree different thicknesses of the air gap. The dependency of thesignals detected by the detector on the defocus, A₂₀, is shown by thegraph in FIG. 4 for air gaps of 40, 100 and 400 nm. The size of thedetector is given by the above equation (A), where the value of NA_(det)was chosen as 0.1 in the calculation. It can be seen that if the valueof A₂₀ is 0, between 90 and 95% of the stray light reflected by therespective air gaps will fall on the detector and reduce thesignal-to-noise ratio. In order to ensure that only approximately 10% ofthe light reflected by these air gaps, of between 40 and 400 nm, fallson the detectors, a defocus value (A₂₀) of 7.5λ is required. On basis ofthis more detailed analysis the criterion for the transparent layerthickness d is:$d \geq \frac{11 \cdot \lambda \cdot n_{cov}}{{NA}_{NF}}$

The maximum thickness of the transparent layer is limited by therespective geometry of the SIL 14 and the optical disk 1 in the opticalscanning device. This maximum value can be estimated as follows.

In use, the SIL 14 is placed on an air-bearing slider schematicallyshown at 40 in FIG. 6. The slider, in one embodiment in which theoptical scanning device is a magneto-optic scanning device, alsoincludes a magnetic coil (not shown). Because of the presence of atransparent layer 26 on the disk, the spot diameter at the exit surfaceof the slider is D. During operation of the scanning optical device, theslider 40 commonly has a ‘pitch’—an inclination with respect to theoptical disk surface. A typical value of pitch is 0.15 to 0.2 mrad. As aconsequence, the air gap thickness for marginal beams at the front andat the rear of the spot is different, causing a difference in near-fieldcoupling efficiency.

Assuming an acceptable air gap thickness difference of 15 nm, D must beless that 100 μm. If the system is to be used for Magneto Optic (MO)readout and carries a thin film coil (not shown) at the bottom of theslider 40, the beam diameter at this surface is limited by the coil'sbore diameter and should be less than 50 μm. This limits the thicknessof the transparent layer 26 (depending on its refractive index) to 15-20μm.

It will be appreciated that a judicious choice of the transparent layerthickness and detector size can cause a significant proportion ofradiation reflected by the air gap to fall outside the effective part ofthe detector 31. A significant proportion is considered to be an amountsuch as to improve the performance of data collection of the scanningoptical device. For example, if at least 50% of the radiation reflectedby the air gap falls outside the effective part of the detector, thiswould improve the data detection ability of the scanning device andhence improve the signal to noise ratio. Preferably, the value of thetransparent layer thickness and the detector size are selected such that90% of the radiation reflected by the air gap falls outside theeffective area of the detector 31.

As described previously, a further problem of such optical scanningdevices is coherent cross talk between the signal layer and the lightreflected by the air gap. It has been found that this cross talk issubstantially independent of detector size. The values of coherentcross-talk from a transparent layer as a function of defocus of light(A₂₀) reflected from three air gaps, again of 40, 100 and 400 nm, on adetector having a radius as defined above have been calculated and theresults are shown in FIG. 5.

FIG. 5 shows that for defocus values (A₂₀) of 2.5λ, 3.5λ and 7.5λ (takenfrom FIG. 4 in a manner described above) and hence transparent layer 26thicknesses of 1.6 μm, 2.3 μm and 4.6 μm respectively (calculated above)the coherent cross talk is significantly reduced.

To have low coherent cross-talk from the air gap it is preferable tohave the transparent layer thickness above the value:$d = \frac{20 \cdot \lambda \cdot n_{cov}}{({NA})^{2}}$

To reduce the coherent cross-talk even further an anti-reflectioncoating can be applied on top of the transparent layer.

So far only diffraction-limited imaging of the spot on the detector hasbeen considered since it allows use of the smallest detector and thus toachieve the best improvement in signal to noise ratio. However, thecurrent invention is not limited to this case. Typically in opticalsystems the spot on the detector is not diffraction limited and has alarger size. The system may have a quadrant detector in combination withan astigmatic servo lens, Focault or spot-size focal error detectionschemes. The size of the spot and thus the detector size is thendetermined by choices made for a particular system, for example by thesize of the circle of least confusion in case of astigmatic focus errordetection or by the size of a defocused spot in case of the spot sizedetection scheme. According to the present invention the spot due to thereflection from the air gap must be sufficiently larger than thedetector size. In particular, in order for 90% of this light to miss thedetector its typical dimension D_(d) must comply with the followingrelation:${D_{d} \leq \frac{2\quad\Delta\quad{l \cdot {NA}_{\det}}}{\sqrt{10}}} = {\frac{4\quad d\quad\left( {NA}_{NF} \right)^{2}}{{\sqrt{10} \cdot n_{cov}}{NA}_{\det}}\quad{or}}$$d \geq \frac{{\sqrt{10} \cdot n_{cov}}{NA}_{\det}D_{d}}{4\left( {NA}_{NF} \right)^{2}}$

For example, for a typical detector size D_(d)=120 μm used in currentoptical heads and NA_(det)=0.1, NA_(NF)=1.4, n_(cov)=1.8 the thicknessof the transparent layer should be d>8.7 μm.

Besides the deterioration of the signal to noise ratio the reflectionfrom the air gap in such a system may cause an apparent shift of thes-curve of the focal error signal. In order to prevent that in anotheraspect of the present invention it is proposed to use an additional setof segments on the detector. Several embodiments of such detectorsintended to work with multilayer optical discs are described in EP 0 777217 A2.

In the above embodiment, the slider is used for providing an air gapsufficiently small to allow evanescent coupling. In another embodiment,a slider is used to provide a low flying height for a magnetic coil in amagnetic field modulation system for a magneto-optical recordable disk.In that case the slider flies at a height of less than 20 μm, preferablyless than 10 μm. More preferably, the air gap is in the region of 1-2μm. In the absence of a transparent layer, the top surface of the diskand the air film in the air gap will be heated locally, possibly leadingto flying instability of the slider. If a lubricant is applied on thedisc surface, it too will be heated, which can lead to its degradation.However, by providing, according to the invention, a transparent layeron the disk of a suitable thickness the localised heating effect can bereduced. The transparent layer should then be greater in thickness thanthe focal depth of the beam, such that the beam where it passes throughthe top surface of the disk is not at its minimum diameter. A lubricantcoating, as described above, may also be added in order to improve thetribology of the interface with the air-bearing slider.

In the slider embodiment the thickness of the transparent layer can bechosen in a wide range from about 2 μm up to 50 μm. It is furtheradvantageous to have it thick enough to avoid coherent opticalcross-talk from the reflection on the top of this layer during read out.In the far-field case if is preferable to select the thickness atbetween 7 and 50 μm. As specific examples, for λ=405 nm and NA=0.85 (atthe optical disk surface) it is about 20-μm; for λ=405 nm and NA=1.4 (atthe optical disk surface) it is about 7-15 μm.

The reduction in localized heating by use of a transparent layer on thedisk is also useful in the near-field case described above; in this casethe thickness of the transparent layer is preferably between 2 and 15μm.

The air-bearing slider used in the above-described embodiments is atleast partly transparent and the characteristics of the radiation beamare altered as the beam passes through it; the slider may carry aplano-convex lens as described above. Alternatively, the slider maycarry two or more lenses of a multi-lens objective, which may be used inorder to achieve higher numerical apertures.

A transparent layer 26, as described above, can be fabricated, forexample, using spin coating of UV curable resin or by sputtering of somehighly refractive material, for example SiO₂/Si₃N₄.

Spin coating produces a smooth layer having a low refractive index, ofthe order of 1.52. This limits the numerical aperture NA to a maximum of1.35 to 1.4, whereas 1.6 or greater is preferable for near fieldapplications. Furthermore, if spin coating is used, an additional hardcoating is preferred.

Sputter coating can produce a transparent layer with a higher refractiveindex, which is sufficiently hard.

It will be appreciated that other suitable materials or processes may beused to fabricate a suitable transparent layer.

It is envisaged that various modifications and variations may beemployed in relation to the above-described embodiments, withoutdeparting from the scope of the invention, which is defines in theaccompanying claims.

1-17. (canceled)
 18. An optical scanning device adapted to be used forscanning an optical record carrier when located in a scanning positionin the device, the optical record carrier including an information layerfor storing data and a transparent layer having a thickness of between 2and 50 μm through which radiation passes to reach the information layer,the device including a radiation source for generating a radiation beam,an objective lens system located in the optical path between theradiation source and the scanning position of the optical recordcarrier, a slider for flying at a height of less than 20 μm above theoptical record carrier and having a transparent portion for transmittingsaid radiation beam, and a radiation detector for detecting radiationreflected by the optical record carrier.
 19. An optical scanning deviceaccording to claim 18, wherein said thickness is approximately 20-30 μm.20. An optical scanning device according to claim 18, wherein saidthickness is approximately 7-15 μm
 21. An optical scanning deviceaccording to claim 18, wherein said slider is adapted to operate with aflying height of approximately 1 to 2 μm, and said thickness is between7 and 50 μm 22-25. (canceled)